The present invention relates to a method for preparing carbon fibers by treating pitch fibers spun from a pitch with a gaseous mixture of air and a gaseous oxidizing agent to be infusibilized, and carbonizing the thus treated pitch fibers into the carbon fibers.
In the preparation of the carbon fibers, the pitch fibers are subjected to a treatment of infusibilization of the pitch fibers before the carbonization thereof. The pitch fibers are made infusible when the pitch fibers are subjected to a take-up system in which the pitch fibers are fed and taken-up around a roll, or a net-belt conveyer system in which the pitch fibers are loaded on and transferred by a net-belt conveyer. However, in the take-up system, a high productivity cannot be obtained since the pitch fibers cannot be taken up at a high velocity due to the low physical strength and ductility thereof. Moreover, it takes much time to mend pitch fibers when it is broken during the reaction according to this system.
In the well-known net-belt conveyer system, the pitch fibers are formed into waves by the net-belt and the fibers are locally deformed by the meshes of the net, because the pitch fibers are not sufficiently infusibilized in the steps in the infusibilizing furnace.
The inventor had found that these problems are solved by providing a bar on the upper part of a tray having a u-type cross section and introducing the tray having the pitch fibers suspended on the bar into the furnace of infusibilization and of carbonization thereby effectively carrying out the infusibilization and carbonization (refer to Japanese Patent Application No. 53-78555/78).
However, in the tray system, the height of suspension of the fiber is restricted by the strength of the fiber, and the packing density of the fiber cannot be made so much larger because of the necessity of preventing the accumulation of heat generated by the infusibilizing reaction and the necessity of uniforming the gas flow for the sufficient replacement of the generated gas in the carbonization than the ordinary value of 1 to 20 kg/m.sup.3. Such an amount of the packing density of the fiber is too small from the economical view. That is, in order to make the capacity of the total facilities, it was necessary to have larger furnaces of infusibilization and of carbonization. This was not desirable from the consideration of raising the production efficiency.
Therefore, it is an object of the present invention to provide an efficient method for preparing carbon fibers from a pitch. This and other objects have been attained by the method of the present invention comprising:
loading the above-mentioned pitch fibers on a net-belt conveyer;
introducing the thus loaded fibers into an infusibilizing furnace having at least two exposing chambers arranged in series, each of the exposing chambers having an atmosphere maintained at a different maximum temperature, thereby raising the temperature of the infusibilizing furnace of a longitudinal direction from the inlet to the outlet of the furnace by steps;
exposing the introduced fibers to a gaseous mixture of air and a gaseous oxidizing agent by passing the gaseous mixture through the introduced fibers at a temperature lower than the softening point of the pitch fibers by 5.degree. to 50.degree. C. on the way of infusibilizing to expose in each of the above-mentioned chambers, thereby infusibilizing the fibers;
introducing the exposed pitch fibers into the carbonizing furnace; and
carbonizing the introduced pitch fibers therein by a flow of an inert gas heated to a temperature of 400.degree. to 1,500.degree. C.